1. Field of the Invention
The present invention relates to systems and methods for logging wells to obtain information concerning the characteristics of underground formations. More particularly, the present invention pertains to nuclear and acoustic well logging sondes, and to logging techniques of measuring the permeability of underground formations.
2. Description of Prior Art
It is well known in the field of production of hydrocarbon fluids from underground deposits that the mobility of such fluids, and, hence, the ease with which they may be obtained, depends in part on the physical structures of the formations in which the fluids are found. Furthermore, the existance of such hydrocarbons in underground formations is itself linked to such formation structure. The fluid in a formation exists within the interstitial spaces of the formation, and the mobility of the fluid through the formation to a production borehole is directly affected by the degree of interconnection of such interstitial spaces as well as the size of such spaces. The ability of a formation to communicate fluid is the formation's permeability, and may be measured by the rate of flow of a fluid in the formation. Thus, fluids are relatively free to move in a formation of high permeability, and have little mobility in a formation of low permeability. Consequently, a measure of the permeability of a particular formation may yield valuable information for determining the extent of the presence of fluids, and the ease with which they may be recovered.
In U.S. Pat. No. 3,890,502 to Dowling et al, a nuclear logging tool is disclosed whereby the permeability of a formation is measured through the mechanism of detecting the flow therein of radioactive chloride ions. The method relies upon the existence of salt water in the formation. An electric field is used to attract chloride ions into the borehole, causing a migration of such particles from the formation. The area is then bombarded with neutrons, and a measure of thermal neutron decay is obtained. The chloride ions are then dispersed back to the formation by a magnetostriction transducer, and a second neutron decay measurement obtained. Since the chloride ions possess large neutron capture cross-sections, the neutron decay data may be expected to reveal the extent of migration of the chloride ions to and from the formation, thus yielding a measure of the formation permeability.
Since the permeability of a formation is directly related to fluid movement, the accuracy of permeability measurements may be enhanced by basing them on measurements of fluid velocity, or volume flow, as fluid is made to move through the formation. A down hole tool, such as a logging sonde, is generally confined by the boundaries of the borehole, and direct measurements by probes into a formation are prohibitive. However, a determination of fluid flow within the borehole may be obtained from a measure of net flow of fluid across the boundary, or interface, between the borehole and the formation itself. Thus, as fluid moves into the formation, a component of fluid flow toward the formation level is established in the borehole. By measuring this component of borehole fluid flow, a determination of the permeability in question may be had.
U.S. Pat. Application Ser. No. 628,174, filed Nov. 3, 1975, now U.S. Pat. No. 4,032,780 discloses a logging sonde designed to measure vertical water flow behind casing lining a borehole. A neutron accelerator is used to irradiate the flowing water with neutrons of sufficient energy to transform oxygen in the water into unstable nitrogen 16 particles. A pair of spaced gamma ray detectors then monitor the radioactive decay of the N.sup.16 particles flowing with the water current. Linear velocity as well as volume flow rate values for the water current may be obtained by appropriately combining the measured radiation detection data.